It's all a weight measure - you would use equal measures of citric acid to water by weight. They're pretty close, though, so you could use a volume measure - but if you do that, then you need to pack down the citric acid. If you were to use 1/3 cup of water and 1/3 cup of citric acid, packed down, combine and dissolve the CA, (may have to use a little heat, I do) - then that's close enough to your 50% measure.
I'm not entirely sure what you're doing wrong. Are you sure you have a 28% sodium chlorite mixture (by weight)? Have you kept it out of UV light since making it? (I keep mine in a cupboard) - My understanding is that citric acid is only good for about 2 years - how old is your citric acid? I'm hoping that Steve will come in here with more suggestions, because I use SC in flakes and I'm thinking you use the powder - and he has more experience with that.. I know that when I make my 28% citric acid mixture, it also starts cloudy when I add the distilled water. I have to dissolve it, again, with heat on an electric burner on low.